1. Field of the Invention
This invention relates to the preparation of a lysable blood platelet reference control for use in a number of routine hematologic determinations. More particularly, the invention is directed to lysable, blood platelet reference controls prepared from animal red blood cells.
2. State of the Art
Stabilized human platelets are commonly used as reference controls for checking the performance characteristics of blood particle counting instruments. The use of human platelets for this purpose, however, is not without its limitations. For example, human platelets are difficult to handle since they are easily activated and tend to aggregate. Human platelets are also very expensive.
Yet another shortcoming of stabilized human platelet reference controls has been experienced with multi-parameter reference controls which contain red blood cells and white blood cells as well as the platelets. In such multi-parameter reference controls, the red blood cells and the platelets mix at different rates, the red blood cells mixing more rapidly, and consequently, it is not uncommon to experience erratic platelet counts when the multi-parameter controls must be resuspended after days of settling.
It is not surprising, therefore, particularly with the advent of automated devices capable of performing multiple hemotogical determinations, to find a growing need for less expensive substitutes useful as blood platelet reference controls. Robert A. Hunt U.S. Pat. No. 4,179,398 describes a blood platelet reference control prepared by shrinking goat red blood cells to the size of human blood platelets and then fixing the shrunken goat blood cells to stabilize them. The shortcoming of the platelet control composition of U.S. Pat. No. 4,179,398 is that the cells shrunken by hypertonic solution as described in the patent tend to return to their original shape. In other words, simulated platelets prepared by U.S. Pat. No. 4,179,398 when put into an isotonic solution often return to their original shape thereby resulting in an inaccurate count.
Another drawback of the platelet control composition of U.S. Pat. No. 4,179,398 resides in the fact the size of the simulated platelets prepared is only the general size of human blood platelets and not the size distribution of human blood platelets. Any system for automated platelet counting which distinguishes human platelets from other cells in the blood on the basis of the characteristic size range and volume distribution of platelets requires that the reference control material closely simulate the size range and volume distribution characteristics of platelets in normal human blood.
Chastain, Jr. et al U.S. Pat. No. 4,264,470 recognizes the problem associated with reference controls of the type aforementioned which do not closely simulate the size range and volume distribution characteristics of platelets in normal human blood and proposes a method for obtaining goat blood cells with the desired size and volume distributions by using different goats, inducing anemia in goats, etc., determining the number and size distribution of the red blood cells in the various samples thus obtained and then blending the red blood cells to gain the appropriate size and volume distributions. The multiple steps and controls necessary to such a prior art process renders it unduly difficult and cumbersome.
Another disadvantage in prior art reference controls for use in blood platelet controls resides in the fact that they are "fixed" or stabilized. When "fixed" platelets are used in multi-parameter controls containing red blood cells, white blood cells and platelets, the platelets may add to the white cell count because "fixed" platelets do not lyse or fragment. Consequently, when a white blood cell count is taken, some of the large platelets are counted in the white blood cell channels giving an inaccurate total white blood cell count.
Wayne Ryan U.S. Pat. No. 4,436,821 describes lysable, permanently shrunk blood platelet reference controls and methods for their preparation; which controls do not return to their original shape when placed in an isotonic solution and closely simulate the size range and volume distribution characteristics of platelets in normal human blood. According to the method of this patent the reference controls are obtained by first fixing animal red blood cells larger than or within the size range of human blood platelets with a fixing agent for red blood cells to partially rigidify the membrane of the cells and then subjecting distinct groups of the partially fixed cells to different osmotic pressures to permanently shrink the cells and provide a mixture that simulates in size, number and volume distribution blood platelets present in human whole blood.
The present invention provides an alternative method of obtaining these lysable blood platelet reference controls described in U.S. Pat. No. 4,436,821.